To date, modern power semiconductor devices, including devices such as power MOSFETs and Insulated Gate Bipolar Transistors (IGBT), have been typically fabricated with silicon (Si) semiconductor materials. More recently, silicon carbide (SiC) power devices have been researched due to their superior properties. III-Nitride (III-N) semiconductor devices are now emerging as an attractive candidate to carry large currents and support high voltages, providing very low on resistance, high voltage device operation, and fast switching times. A typical III-N high electron mobility transistor (HEMT), shown in FIG. 1, comprises a substrate 10, a channel layer 11, such as GaN, atop the substrate, and a barrier layer 12, such as AlxGa1-xN, atop the channel layer. A two-dimensional electron gas (2DEG) channel 19 is induced in the channel layer 11 near the interface between the channel layer 11 and the barrier layer 12. Source and drain electrodes 14 and 15, respectively, form ohmic contacts to the 2DEG Gate electrode 16 modulates the portion of the 2DEG in the gate region, i.e., directly beneath gate electrode 16.
Field plates are commonly used in III-N devices to shape the electric field in the high-field region of the device in such a way that reduces the peak field and increases the device breakdown voltage, thereby allowing for higher voltage operation. Examples of field plated III-N HEMTs are shown in FIGS. 2 and 3. The device in FIG. 2 includes a field plate 18 which is connected to gate electrode 16, i.e., a gate-connected field plate, and an insulator layer 13, such as a layer of SiN, is between the field plate and the barrier layer 12. Field plate 18 can include or be formed of the same material as gate electrode 16. The manufacturing process for a device with a gate-connected field plate is typically relatively simple as compared to that for devices with different field plate configurations, since there is no need to form separate field plate and gate electrode layers, thus the deposition of the gate electrode and field plate can be performed in a single processing step. However, the gate-connected field plate 18 in FIG. 2 increases the capacitance between the gate 16 and drain electrodes 15, thereby reducing the effective operating speed of the device. This increase in capacitance between the input and output of the device, along with the corresponding reduction in high frequency response, is known as the Miller capacitance effect or Miller effect. For applications in which the III-N HEMT shown in FIG. 2 is used, the source electrode 14 is typically grounded, as indicated in the figure.
In the device shown in FIG. 3, the field plate 18 is connected to the source electrode 14, i.e. field plate 18 is a source-connected field plate. Connecting the field plate to the source electrode can reduce or eliminate the Miller effect, since the voltage on the field plate remains fixed when an input signal is applied to the gate electrode. For this configuration, the capacitance between input and output is the source-to-drain capacitance, which typically is small and therefore has a negligible impact on device performance. However, the manufacturing process for this device is more complicated than that for devices with gate-connected field plates, such as the device of FIG. 2.